DNA of nptII and/or nptIII positive soil sampleswas concentrated
by vacuum centrifugation of 100 ml of the original PowerSoil eluate
for 30 min. The dried pellet was resuspended in 20 ml of H2O
(molecular biology grade; Sigma) and stored at 20 C for further
analysis. Two ml of the concentrated solution were used as template
for the quantitative nptII/nptIII TaqMan assay. Absolute quantification
was performed using semi-logarithmic dilutions of pCR2.1
plasmids containing either nptII (GenBank accession number
V00618) or nptIII (V01547) full length wild type gene inserts as
external standards. A check for PCR inhibition was performed with
1:5 and 1:25 serial dilutions of the template. Sample eluates which
showed inhibition were re-purified with the PowerClean Kit
(MoBio Laboratories) according to the manufacturer's instructions.
Copy number calculations were performed using the second derivative
maximum algorithm provided by the LightCycler LC480
software package (version 1.5) with default parameters. The samples
were tested at least in triplicates.